1. Field of the Invention
The invention relates to a process for balancing a video signal, and more particularly, to white balancing a video signal, and also to image processing equipment for carrying out the process.
2. Discussion of Relevant Art
Processes of the above-mentioned kind are known in great number and variety in the art, for example, under the term xe2x80x9cwhite balancingxe2x80x9d. The white balancing processes are usually used to balance a video signal so that a white subject always produces a video signal that corresponds to a defined white signal in the respective video standard, completely independently of the illumination source that illuminates the white subject Such video standards are, for example, PAL or NTSC, in which the color white is fixedly defined.
Many different circuits are known from the state of the art for carrying out white balancing, and carrying out the white balancing fully automatically or semi-automatically.
In a process for semi-automatic white balancing, described by way of example, a white surface, such as for example a white sheet of paper, is positioned in front of a video camera and, on pressing a button, an amplification factor is determined for each of the three color channels red, green and blue, such that the three signals are of equal magnitude after amplification, and the balanced video signal produces a white color on a monitor. The amplification factors that are determined are then stored in a corresponding memory for further use of the video camera, and remain constant until the next white balancing of the video camera.
In contrast to semi-automatic white balancing, in fully automatic white balancing the amplification factors are not determined only once by a white balancing, but are continuously adjusted during picture taking. In a process described here by way of example, the video camera has a sensor for this purpose, in the neighborhood of the objective that constantly measures the spectral distribution of the illumination source and continuously derives amplification factors from the spectral distribution of the illumination source that are then used for the corresponding amplification of the individual color components of the video signal.
The characteristic feature of conventional white balancing devices is that a video signal is produced, completely independently of the kind of illumination of a white subject, and corresponds to the defined white signal of the video standard that is used.
The object of the invention is to provide a process for balancing a video signal, and a corresponding equipment for carrying out the process, which can be used with particular advantage in the field of medical technology.
This object of the invention is achieved by a process comprising illuminating a white surface with a selected illumination source, balancing the video signal in such a manner that the color effect of an image of the white surface displayed by a monitor, corresponds as closely as possible to the color effect that results from direct observation by the human eye of the white surface. The image processing device comprises an imaging device that produces a video signal of photographed subjects, a balancing unit that balances the video signal such that the color impression of an image displayed by a monitor of a white surface photographed by the image device and illuminated with a selected illumination source corresponds as closely as possible to the actual color impression of the white source resulting from direct observation by the human eye, based on the selected illumination source.
An important concept of the invention is that the video signal is balanced in such a manner that the color effect of an image of a white or colored surface shown by a monitor corresponds as closely as possible to the color effect of the photographed surface, taking into account the color temperature of the illumination source used.
This provides the particular advantage that the images of subjects taken by the video camera are displayed on the monitor exactly the same as they would appear on direct observation of the corresponding subjects depending upon the respective illumination source. This is particularly desirable in many fields of medical technology because a substantially more realistic impression of the image arises for the observer of a corresponding video image. This can be of great advantage when details of an operation are photographed with a video camera and an observer has to quickly find his way around the photographed images.
The balancing of the video signal preferably takes place in that, in a manner known per se, the video signal is white balanced in a first step, and additionally a correction signal is determined, which corresponds to the kind of illumination source that illuminates the white surface, and based on which the white balanced video signal is corrected.
This has the particular advantage that a video camera that can be purchased as standard equipment with a white balancing unit built into it can be used for carrying out the process according to the invention, and only a correction unit has to be provided in addition, which determines the correction signal.
The white balancing can be carried out in various ways that are already sufficiently well known from the state of the art
The determination of the kind of illumination source can also take place in various ways. One possibility could be that several different illumination sources are connected directly to the video camera by means of an electrical lead, and that on switching on one of the illumination sources a corresponding signal is furnished to the video camera, and based on this signal the video camera determines the correction signal. Another possibility would be, for example, to provide in the video camera a sensor that can determine the spectral distribution of the illumination source. Based on the results of the sensor, the exact illumination source is determined and the corresponding correction signal is determined.
In a particularly simple embodiment, the kind of illumination source is determined from the non-balanced video signal. This embodiment has the advantage that the connecting leads to the illumination source, or the sensor for measurement of the spectral distribution of the illumination source, can be dispensed with. The kind of illumination source can be determined from the difference amounts of different color components of the non-balanced video signal.
After the kind of illumination source has been determined, the value of the correction signal corresponding to the illumination source that was determined can be read out from a memory.